Method and apparatus for conveying information signals

ABSTRACT

Video signals from a source are digitized and applied to a source coder which segregates the signals into four channels (CH1 to CH4) of progressively less visual significance. CH1 carries basic picture information at a relatively low resolution. CH2 to CH4 carry differential information at progressively higher resolutions. The four channels signals are applied to respective coders and the digital outputs are combined in a matrix with voltage levels falling off as visual significance decreases. Received signals, transmitted say by way of a satellite, are applied to a first, high level decoder which extracts the digital waveform for CH1 as DCH1. This is recoded and subtracted from the input to provide a signal to a decoder which recovers CH2 as DCH2, and so on. DCH1 to DCH4 are applied to a source decoder complementary to the coder to recover the source signal. Low significant channels can be omitted at the transmitter, or not be recovered at the receiver to adapt to the prevailing signal to noise performance.

The present invention relates to a method and apparatus for conveyinginformation signals, particularly (but not exclusively) televisionsignals. In general television signals are conveyed from a source to areceiver by way of a medium. The medium may be a transmission medium(e.g. terrestrial or satellite radio, cable) or a storage medium (e.g.video cassette, optical disc). Many different sources are now availableincluding studio and portable cameras, computers, tele-cine apparatus,and so on. There is a concomitent tendency for standards to proliferateif only to match the different bandwidths and/or signal to noiseperformances of different systems. Such proliferation is clearlyundesirable.

A related problem arises in that it is often not possible to specify thebandwidth or signal to noise performance which is appropriate in a givensystem. There may vary in dependence upon fluctuating transmissionmedium characteristics, size of receiving antenna in the case ofsatellite radio transmission, changes in transmitter power, and so on.For brevity, the remaining description refers simply to bandwidth,although it is frequently the signal to noise performance which isactually the determining factor.

The object of the present invention is to provide a solution to theproblems which thus arise, by means of a method and apparatus whichallow adaptable standards to be set up within a universal system.

According to the invention in one aspect, there is provided a method ofconveying television signals from a source to a receiver by way of amedium, wherein signals from the source are segregated into a pluralityof channels corresponding to progressively less visually significantinformation, a composite signal is formed, for conveyance by the saidmedium, by a reversible combination of component signals from the saidchannels with the powers of the component signals varying directly independence on the significances of the channels, and wherein, at thereceiver a number of the component signals are recovered from thecomposite signal conveyed to the receiver by the said medium and atelevision signal is formed from the recovered signals.

Such a method may be employed in the content of a heirarchy ofstandards, comprising a prime, maximum bandwidth standard and otherstandards requiring progressively less bandwidth, including a basic,minimum bandwidth standard. When the prime standard is appropriate, allthe channels are employed at the source but, when lesser bandwidths arerequired, channels are progressively dropped, starting from the leastsignificnt channel. The conveyed signals can include a label whichinforms the receiver as to the number of channels which have beenemployed.

The receiver can recover all of the component signals included in theconveyed composite signal. However the receiver does not have to recoverall of the component signals. If for example the medium is noisy for anyreason (fringe area reception, small receiving antenna, and so on) thereceiver may ignore at least the least significant component signalincluded in the conveyed signal. This will enhance the received pictureby cutting out displayed noise, although the picture will naturally havereduced detail.

The invention will be described in more detail, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a diagram showing how different pixel resolutions are employedin a television frame,

FIG. 2 is a diagram showing how a coarsest resolution pixel isprogressively subdivided,

FIG. 3 is a block diagram of apparatus embodying the invention, and

FIG. 4 is a graph used in explaining receiver performance in directbroadcasting by satellite.

In order to allow a common transmitting and receiving system for allstandards, it is desirable that the higher bandwidth standards are allsubsets of the preceding standard. In particular, in the spatial domain,each standard should employ a finer sampling grid than the precedingstandard but which contains the sampling points of the precedingstandard.

The invention in various other aspects and advantageous developments ofthe invention are defined in the appended claims.

The invention will be described in the context of a heirarchical systemof standards set out in the Table below. The left half of the Tablegives the number of pixels per screen for six different spatial samplinglevels denoted by n=0 to n=5, with progressive subdivision of thesampling grid. The right half of the Table gives the consequent pixelrate for each of four different temporal sampling levels denoted by m=1to m=4.

    ______________________________________                                                          m                                                                             1    2      3       4                                                         Frames/second                                                                 12.5 25     50      100                                     ______________________________________                                        PIXELS                  PIXELS/                                               n   Horizontal                                                                              Vertical Frame  SECOND  (Millions)                              ______________________________________                                        0   128       64       8.2  k   0.1  0.2  0.4   0.8                           1   256       128      33   k   0.4  0.8  1.6   3.3                           2   512       256      130  k   1.6  3.3  6.6   13                            3   1024      512      520  k   6.6  13   26    52                            4   2048      1024     2    M   26   52   105   209                           5   4096      2048     8.4  M   105  209  419   838                           ______________________________________                                    

FIG. 1 shows the pixel structure. Crosses represent the pixels at thehighest resolution, i.e. n=5 in the above example. Progressively largercircles pick out the pixels in the progressively lower resolutionscreens. FIG. 1 does not indicate pixel sizes. FIG. 2 shows one pixel Pat the lowest resolution (n=0) partitioned into four pixels P₁ to P₄ atthe next resolution, each partitioned into four pixels P₁₁ to P₁₄, P₂₁to P₂₄, etc. For simplicity, further partitioning is not shown.

If the symbols P etc are now taken to denote the information content ofthe pixel (e.g. digital R, G and B or Y, U and V values), the n=0 frameis fully defined by the set of 8.2k P values. The n=1 frame can be fullydefined by these values plus 33k differential values, where the fourdifferential values for every P are:

    DP.sub.1 =P.sub.1 -P

    DP.sub.2 =P.sub.2 -P

    DP.sub.3 =P.sub.3 -P

    DP.sub.4 =P.sub.4 -P

This procedure can be continued in a heirarchical fashion so that wehave

    DP.sub.11 =P.sub.11 -P.sub.1

    DP.sub.12 =P.sub.12 -P.sub.1

and so on.

This heirarchical structure makes possible an advantageous method oftransmission in accordance with the invention, whereby a first channelis used to carry the P information, a second channel is used to carrythe DP₁₋₄ information, a third channel is used to carry the DP₁₁₋₄₄information, and so on, with progressively decreasing powers in acomposite signal.

So far as temporal resolution is concerned, if frames are alwaysregarded as nominally occurring at 100/second, every frame is used form=4, every second frame for m=3, every fourth frame for m=2 and everyeighth frame for m=1. The heirarchical approach explained above inrelation to the spatial domain can be extended to the temporal domain.

Let P₀ (T₁) denote the basic set of pixel values for every eighth framewith n=0 and m=1. The set of pixel values for the intervening frameswhen n=0 and m=2 are P₀ (T₂) and these are represented as:

    P.sub.0 (T.sub.2)=P.sub.0 (T.sub.1)+DP.sub.0 (T.sub.2),

i.e. the intervening frames are given by the values for the basic framesplus differential values DP₀ (T₂), which are transmitted in a channelassigned thereto.

If all the pixel rates in the above Table are considered, it will beseen that up to 20 different bandwidths can be established by use of abasic channel carrying the n=0, m=1 information and 19 differentialchannels carrying differential data pertaining to progressively higherspatial and temporal resolutions. FIG. 3 illustrates an apparatus whichcan be used in the transmission and reception of such segregatedinformation.

FIG. 3 shows a source 10 of a video signal feeding an analog to digitalconverter 12 operating at a selected top pixel rate, e.g. the highestrate of the above table. The digital values are fed to a source coder 14which comprises a multiple frame digital store and hardware/softwaremeans for forming differences such as to form on channel CH1 the basicpixel information P₀ (T₁) and on channels CH2 etc (for simplicity onlyfour channels CH1 to CH4 are shown) the differential informationpertaining to the higher temporal and spatial resolutions.

Each channel feeds a corresponding coder 16 which codes the informationinto a pulse waveform in accordance with a suitable pulse code. Suitablepulse codes for transmitting digital television are known in themselves.The coders feed amplifiers 17 with progressively smaller gains (A1 toA4). The outputs of the amplifiers, which are at progressively smalleramplitude levels, as symbolized by the adjacent waveforms, are combinedin a summing network 18 to form a composite signal to be conveyed. Byway of example, FIG. 3 shows the signal being conveyed via a transmitter20, satellite 22 and receiver 24 with receiver dish antenna 26. Theinjection of receiver noise is symbolized by a noise source 28 and mixer30.

The received signal is applied to a decoder 32 for channel 1 with alevel set by an amplifier 34, such that the decoder 32 only extracts thedata for the most significant channel, denoted DCH1. The amplifier 34thus has a gain which, symbolically at least, is 1/A1.

The DCH1 data is re-coded by a coder 36 identical to each of the coders16 and the output of this coder is subtracted in a mixer 38 from theoutput of the amplifier 34 to create a signal containing only thedifferential information of channels CH2, 3, 4. This signal is amplifiedto a suitable level by an amplifier 40 such that a decoder 42 canextract the decoded channel 2 data DCH2. The circuitry is repeated, ascan be seen in FIG. 3 to extract DCH3 and DCH4.

The decoded signals DCH1-4 are applied to a source decoder 44 whichcomplements the source coder 14 and recreates the video signal by aprocess of successive interpolations. For example, since DP₁ =P₁ -P isthe equation used to form DP₁, on reception P₁ can be recovered usingthe equation P₁ =P+DP₁.

Note that, although there may be a universal standard catering for allpossible resolutions (bandwidths), in any given implementation it willobviously be sensible to provide only that amount of hardware consistentwith the available and desired bandwidth. For smaller bandwidthapplications, a reduced number of channel coders 16 will be employed.The data format, in particular the number of channels CH1, CH2, etc ispreferably transmitted as a label, a technique too well known in itselfto require description here.

At the receiving end there may be the same number of decoders 32, 42 etcas there are coders 16 but if there is a lot of receiver noise it willbe better to omit one or more least significant decoders. If too manydecoders are active the least significant ones will be affected by thenoise from the source 28 and will produce visible noise. The controlover the number of receiver channels may be a matter of receiver designfor an intended application, by way of presets adjusted to suit aparticular installation site or by way of an operator control. However,it will be possible to design "intelligent" receivers capable ofdecoding the labels and displaying pictures originated on all levels ofthe heirarchy, although with a resolution limited in each case by thedisplay device and processing.

The compatibility between the different levels of the hierarchy can beachieved by suitable choices of sampling structures and coding, suchthat each level contains all the data from those levels beneath it. Thushigher levels could use a finer sampling grid, but one that alsocontains the sampling points of lower levels as in the example alreadydescribed.

The sampling structures could also be selected to accommodate existingpicture origination and display standards e.g. PAL, NTSC, film,NHK-HDTV.

The advantages of this approach are considerable. Many different sourcesignal formats could be used with a single display device and with acommon interface. In contrast, within the present PAL environment thereis already a choice of baseband or UHF-composite, YUV or RGB signals.Many computers and work-stations use their own individual standards.

Having established the heirarchical principle, improved systems could beadopted without the problem of compatibility with existing equipment orthe delays of agreeing further standards.

From the outset, those broadcasters with higher capacity channels couldsupport higher quality services. Conversely, those services that did notrequire a high-quality picture, e.g. local community TV, could implementlower-quality systems and yet still be received by those viewers withhigh quality displays, but whose receivers have the intelligenceexplained above.

Receiver and source-equipment manufacturers could support the standardsthey chose with a guarantee of compatibility with other equipment.Individual parts of the chain could be upgraded as permitted byadvancing technology.

As the number of broadcast services increases and as technology givesrise to higher definition picture sources and displays, it seems likelythat there will be increasing demands on the available RF spectrum. As aresult, any limitation in the received piture quality of a futuretelevision system, will probably be imposed by the broadcast channel. Itis therefore important that any future broadcast transmission codeshould make efficient use of the available channel capacity.

Various advanced digital coding schemes have already been proposed forsatellite applications. However, these provide a fixed data rate over awell defined channel. For broadcast use, the requirements are different.The characteristics of the channel will vary, both from receiver toreceiver (because of differing dish sizes and geographical locations)and with time (satellite transmissions will probably increase in poweras time progresses).

One of the drawbacks of direct digital transmission is the limitationimposed by the worst-case signal reception. With a simple p.c.m. linkcarrying its maximum data capacity (where this is limited by gaussiannoise) a small variation in carrier-to-noise ratio will cause a largevariation in error rate. As more advanced coding techniques are used(which allow the information limit of the channel to be approached moreclosely), this threshold region, between satisfactory and unsatisfactoryreception, becomes narrower still. The data capacity of the transmissionsystem is therefore limited to that achieved under worst-caseconditions, so penalising the majority of subscribers. In contrast, ananalogue transmission link provides the maximum available informationcapacity under prevailing conditions; for example, an improvedtelevision carrier-to-noise ratio results in an improved picturesignal-to-noise ratio. However, the user may not be in a position totake full advantage of such improved channel capacity; for example, hemay not be able to see the improved noise performance. Instead he mayprefer better resolution or movement portrayal if these were available.This kind of overall optimisation of the use of a variable capacitychannel is difficult to achieve with analogue modulation techniques butcan be achieved using digital signals.

Thus, the apparatus described with reference to FIG. 3 employs amodulation process which provides an improved data throughput withimproved channel signal-to-noise ratio. The extra data bits so gainedare used to fill in fine detail, to provide more bits per pixel orprovide more frames per second. The extra data is transmitted at a lowerpower, such that it does not interfere with the decoding of the moresignificant data. There are several layers of reducing significance,where the number of layers is limited by the best expectedsignal-to-noise for the channel.

This technique can be applied to the direct broadcasting of televisionby satellite. Here, the channel capacity is determined primarily by thereceiving dish size, the geographical location and the satellitetransmission power. Taking a WARC satellite channel as an example, thetheoretical maximum data capacity for a 20 MHz channel at 14 dB s/n (0.5m dish aerial, worst case reception) is 94 Mbit/s. FIG. 4 plots datacapacity against signal to noise ratio. Assuming a capacity of only 0.6of the maximum where possible in practice, the data capacity wouldbecome 56 Mbit/s. On this basis, and assuming the received signal powerto be proportional to the area of the receiving dish, a portable 0.25 mdish should receive 34 Mbit/s and a large 2 m dish 100 Mbit/s. With a 15dB improvement in transmission power from the satellite and use of thefull 27 MHz bandwidth, a data rate of 200 Mbit/s should be possible.

In this way, those viewers wishing to invest in large dish aerials couldreceive a high definition service while a compact portable set couldprovide a picture quality comparable with the present PAL system. Asreceiver technology advanced and satellite transmissions became morepowerful, picture quality would automatically improve.

As indicated above, the invention is not limited to use with televisinsignals. In the case of audio, for example, the less significantinformation may relate to higher audio frequencies.

I claim:
 1. A method of conveying television signals from a source to areceiver by way of a medium, comprising the steps of segregating signalsfrom the source into component signals in a corresponding plurality ofchannels corresponding to progressively less visually significantinformation, forming a composite signal, for conveyance by the saidmedium, by subjecting the component signals from the said channels to acombining operation to form a composite signal with the powers of thecomponent signals in the composite signal varying directly in dependenceon the visual significance of the information in the channels, thecombining operation being such as to be reversible to recover thecomponent signals from the composite signal, and, at the receiver,recovering a number of the component signals from the composite signalconveyed to the receiver by the said medium and forming a televisionsignal from the recovered signals.
 2. A method according to claim 1,wherein the number of recovered signals is less than the number ofchannels.
 3. A method according to claim 2, wherein the number ofrecovered signals is adjusted in dependence upon at least one of thebandwidth and/or signal to noise performance of the medium, thebandwidth and/or signal to noise performance of the receiver and theresolution of a display device of the receiver.
 4. A method according toclaim 1, wherein the number of channels is variable and the compositesignal includes a label indicating at least the number of channels inthe signal conveyed to the receiver.
 5. A method according to claim 4,wherein the receiver is responsive to the label to recover the number ofcomponent signals indicated by the label.
 6. A method according to claim1, wherein the component signals are digitally coded and are additivelycombined in the composite signal with amplitudes varying directly independence on the significances of the channels.
 7. A method accordingto claim 6, wherein the component signals are recovered from thereceived composite signal in accordance with components of the compositesignal in different amplitude ranges.
 8. A method according to claim 1,wherein the channels correspond to different levels of a heirarchy ofincreasing spatial and/or temporal resolution.
 9. A method according toclaim 8, wherein there are a plurality of standards in the spatialdomain, each employing a finer sampling grid than the preceding standardbut which contains the sampling points of the preceding standard.
 10. Amethod according to claim 9, wherein each finer sampling grid has twiceas many pixels in the horizontal direction and/or in the verticaldirection as the grid of the preceding standard.
 11. A method accordingto claim 1, wherein the most significant channel carries signalsdefining a basic minimum bandwidth picture and each other channelcarries signals representing differences relative to the precedingchannel.
 12. A method of conveying television signals from a source to areceiver by way of a medium, comprising the steps of segregating signalsfrom the source into component signals in a corresponding plurality ofchannels corresponding to progressively less visually significantinformation, forming a composite signal, for conveyance by the saidmedium, by subjecting the component signals from the said channels to acombining operation to form a composite signal with the powers of thecomponent signals in the composite signal varying directly in dependenceon the significance of the information in the channels, the combiningoperation being such as to be reversible to recover the componentsignals from the composite signal, and, at the receiver, recovering anumber of the component signals from the composite signal conveyed tothe receiver by the said medium and reconstituting an information signalfrom the recovered signals.
 13. Apparatus for transmitting informationsignals from a source thereof, comprising first coding means whichsegregate the information signals into a plurality of channels whichcarry progressively less significant information, second coding meanswhich code the signals in the channels into respective coded signalswith power levels varying directly with said information significance,and combining means which combine the coded signals into a compositesignal in accordance with a combining operation such as to be reversibleto recover the component signals from the composite signal. 14.Apparatus according to claim 13, wherein the second coding meanscomprise a plurality of digital coders individual to the channels andamplifiers on the outputs of the coders having gains varying directlywith significance.
 15. Apparatus according to claims 13 or 14, whereinthe first coding means provide a basic information signal in a first ofthe channels and a series of differential information signals in theother channels, each representing a difference from the informationconveyed via the preceding channel(s).
 16. Apparatus for recovering aninformation signal from a received composite signal including componentswith different power levels corresponding to different levels ofinformation comprising a series of decoding means operative to detectand decode successively lower levels of information in the receivedcomposite signal in accordance with the power levels thereof, andfurther decoding means operative to decode the signals from the seriesof decoding means into the information signal.
 17. Apparatus accordingto claim 16, wherein the series of decoding means comprise a firstdecoding means operative to detect and decode a high level component inthe received signal and an ordered plurality of second decoding means,each of which is operative to detect and decode a corresponding levelcomponent in a residual signal, and means for forming the residualsignal comprising means for re-coding the component detected and decodedby the preceding decoding means and means for subtracting the re-codedsignal from the input to said preceding decoding means.
 18. Apparatusaccording to claim 17, wherein the further decoding means creates theinformation signal from the decoded high level component successivelyinterpolated in the spatial and/or time domain in accordance with theother decoded components decoded by the second decoding means.
 19. Amethod of recovering information from a signal containing codedinformation at different amplitude levels, comprising the steps ofdecoding the signal at a succession of decreasing signal amplitudelevels n=1, 2 and so on such that, at each level n the coded informationfor subsequent levels n+1 and so on does not influence the decoding, andre-coding the resultant decoded signal at each level and subtracting there-coded signal from the signal which was decoded at the level n to formthe signal which is decoded at level n+1.